1. Field of the Invention
The present invention relates to a developing unit applicable to machines such as copiers, printers, facsimile machines which perform image forming by electrophotography. More particularly the present invention relates to a developing unit which regulates the developer supplied on the developer support so as to form a developer layer of a predetermined thickness having a specified amount of static charge by means of a flexible plate-like developer layer regulating member and conveys the developer layer to a static latent image to develop it into a visual image.
2. Description of the Related Art
Conventionally, a two-component developing unit using a so-called two-component developer consisting of a toner and a magnetic carrier is disadvantageously complex, large-sized and expensive because it needs a toner concentration regulating device to keep the mixing ratio of the toner to the magnetic carrier constant. On the other hand, a mono-component developing unit using a so-called mono-component developer consisting of only a toner with no magnetic carrier advantageously has a simple structure and hence can be made compact and also offers cost and maintenance benefits.
Mono-component developing units can be roughly classified into two types, that is, the magnetic mono-component developing unit using a mixture of resin and magnetic iron powder or using a magnetic toner of resin having magnetic iron particles as cores, and the non-magnetic mono-component developing unit using a non-magnetic toner composed of resin and pigments with no magnetic material.
Two systems have been known as the magnetic mono-component developing units: the first is the non-contact developing system in which development is performed by conveying the magnetic toner by means of a non-magnetic developing sleeve having a magnet roller therein while an alternating electric field is applied across the space between the developing sleeve and the photosensitive drum set with a predetermined gap therebetween so as to cause the toner to jump in a reciprocating manner; and the second is the contact developing system in which development is performed by forming a large toner brush so as to come into contact with the static latent image on the photosensitive drum surface. The former needs a developing bias of a d.c. voltage with a superimposed a.c. voltage, and hence requires a complicated power source and that the developing unit be resistant to high voltage. On the other hand, the latter can make do with only a simple d.c. developing bias power source but produces a Hi-gamma (.gamma.) image with poor gradation.
There are two systems for the non-magnetic mono-component developing units: the first is the non-contact developing system in which development is performed by applying an alternating electric field across the space between the developing roller and the photosensitive drum set with a predetermined gap therebetween so as to cause the toner to jump in a reciprocating manner; and the second is the contact developing system in which development is performed by bringing the conductive elastic developing roller into contact with the photosensitive drum. The former needs a developing bias of a d.c. voltage with a superimposed a.c. voltage, and hence requires a complicated power source and that the developing unit be resistant to high voltage. On the other hand, the latter needs only a simple d.c. developing bias power source. Since non-magnetic mono-component developing units using a toner having no magnetic material and do not use any magnet roller, they offer the advantage of providing a compact and inexpensive color developing unit which produces clear colors.
For all the mono-component developing units, it is necessary to form a tribo-charged developer (toner) layer having a predetermined layer thickness on the developing roller or developing sleeve.
As the method for forming a developer (toner) layer on the developing roller, Japanese Patent Publication Sho 63 No.16736 and Japanese Patent Publication Hei 4 No.73152 disclose devices having an elastic regulating plate or a plate-like flexible tribo-charging element. Japanese Patent Application Laid-Open Sho 62 No.182780 disclose a developer layer regulating member which has a metal sheet having spring properties and a soft elastic part integrated with this metal sheet and located between the metal sheet and the developer support.
In order to form a uniform developer layer over a long period, Japanese Patent Publication Hei 6 No.93152 limits the ratio of the free length to the thickness of the leaf spring of a developer regulating member.
Typically, a plate-like developer layer regulating member having flexibility (hereinbelow referred to as `elastic blade`) can be modeled as a cantilever 335 supported at a fixed supporter 355 as shown in FIG. 1. The relationship between an abutment load P(kgf) acting on cantilever 335 shown in FIG. 1 at its free end and its deflection amount .delta.(mm) is represented by the following formula (1): EQU P=(3EI/L.sup.3).delta. (1)
where
E: elastic modulus(kgf/mm.sup.2) PA1 I: geometrical moment of inertia(mm.sup.4) PA1 L: cantilever free length (mm) PA1 .gamma.: mass per unit volume (kg/mm.sup.3) PA1 A: cross section of the cantilever (mm.sup.2) PA1 a developer support for retaining and conveying the developer; and PA1 a flexible plate-like developer layer regulating member which is supported and fixed at one end and is free at the other end and abuts a flat portion of itself in proximity to its free end against the developer support so as to form a developer layer of a designated thickness on the developer support, and is characterized in that void space made up of at least one through-hole and/or recess is formed along the width in the area between the abutment portion of the developer layer regulating member abutting on the developer support and the fixed supported end. PA1 a developer support for retaining and conveying the developer; and PA1 a flexible plate-like developer layer regulating member which is supported and fixed at one end and is free at the other end and abuts a flat portion of itself in proximity to its free end against the developer support so as to form a developer layer of a designated thickness on the developer support, and is characterized in that a plurality of void spaces made up of through-holes and/or recesses are formed along the width in the area between the abutment portion of the developer layer regulating member abutting on the developer support and the fixed supported end, and the plurality of void spaces are formed of the same void space shape at regular intervals, on the area of the developer layer regulating member corresponding to at least, the middle, the developer layer forming area on the developer support.
From the above formula (1), it is understood that the abutment load P at the free end is proportional to the free end deflection amount .delta. and is inversely proportional to the cube of the cantilever free length L when the material and the cross-section shape of the cantilever are constant (EI=constant).
In designing an elastic blade, the material, free length L, plate thickness t and free end deflection amount .delta. are determined based on the abutment load P at the free end, required for the toner layer to be properly formed on the developing roller. Here, since the free end deflection amount .delta. of the elastic blade is determined by the attachment position of the elastic blade with respect to the developing roller, the positional attachment error of the elastic blade will appear as the error of the free end deflection amount .delta. and hence as an error of the abutment load P at the free end.
Further, eccentricity, run-out of the developing roller, etc. will cause variation in the abutment load P at the free end. Wear at the developing roller bearing and thermal expansion and contraction of the supporter member for the developing roller and the elastic blade, that is, the developing unit casing, also will cause the abutment load P at the free end to vary with the passage of time (lapse of time) and also dependent on the environmental conditions.
The margin of the positional attachment error or the installation tolerance of the elastic blade can be enlarged by reducing (3EI/L.sup.3), the constant of proportionality between the free end deflection amount .delta. of the elastic blade and abutment load P at the free end. In general, the free length L of the elastic blade is determined by the size and configuration of the developing unit so that it is difficult to make it greater than the developing unit needs. Further, in order to make the developing unit compact, the free length L inevitably shortens, which further increases the constant of proportionality (3EI/L.sup.3),
Since elastic coefficient E is a characteristic value of the material of the elastic blade, the choice is limited. Therefore, in order to make the constant of proportionality, (3EI/L.sup.3) small, it is more realistic to reduce the value of the geometrical moment of inertia I.
Meanwhile, the fundamental circular frequencies .omega..sub.n of cantilever 335 shown in FIG. 1 are represented by the following formula (2): ##EQU1##
From the above formula (2), it is understood that the fundamental circular frequencies .omega..sub.n of cantilever 335 are proportional to the square root of the geometrical moment of inertia I when the material and the free length of the cantilever are constant.
When the elastic blade receives vibrations at its free end due to the eccentricity, and/or run-out and the rotation of the developing roller or due to stick-slip caused by variations of its frictional resistance with the developing roller, the elastic blade may resonate and exert adverse influence on toner layer formation.
In order to avoid the elastic blade from resonating at its fundamental circular frequencies .omega..sub.n, it is preferable to control the geometric moment of inertia I since the flexibility in selection of the free length L and the elastic coefficient E is limited as stated above.
The geometrical moment of inertia I is given by formula (3): EQU I=Ht.sup.3 /12 (3)
where t(mm) is the plate thickness of the elastic blade, H(mm) is the full width.
In the above formula (3), the full width H of the elastic blade is determined as a constant value depending upon the developer layer forming width. Therefore, in order to reduce the geometrical moment of inertia I, it is necessary to make the plate thickness t of the elastic blade small. However, there is a limitation from the viewpoint of the manufacturing process of the blade or from a handling viewpoint. That is, it is necessary to make the geometrical moment of inertia I small without reducing the plate thickness t of the elastic blade more than necessary.
However, with regard to conventional elastic blades, the geometrical moment of inertia I can be changed by only the plate thickness t. Therefore, there has been little design flexibility to increase the margin for the abutment load, also, taking into consideration the variations of the passage of time and change with environmental conditions and to select the fundamental circular frequencies at which the toner layer formation failure due to resonance can be inhibited.